There is an ever increasing convergence of the media industry (including television, video, three dimensional graphics, electronic publishing, and entertainment), the computer industry (including desktop computing, personal computers connected by local area networks, electronic mail, web sites etc.), and the telecommunications industry (both fixed and wireless communications networks). All of these converging industries rely on high-speed data communication capabilities.
High-speed data communication is particularly important for Internet communications. The Internet offers access to an extraordinary variety of information resources across the world. Typically, users make that access from a fixed location, such as their home, business, or school. However, cellular telephones, coupled with an increasing variety of other wireless devices, such as wireless laptops and personal digital assistance (PDAs), are changing otherwise fixed points of access to the Internet to include mobile access by these types of mobile terminals. For the sake of simplicity, the term “mobile terminal” is used to encompass all types of wireless devices.
Mobile radio packet data communications employ a different model than the circuit-switched model used, e.g., for traditional mobile radio voice communications. In circuit-switched communications, each communication link is allocated a dedicated radio channel, i.e., a frequency in an FDMA system, a time slot in a TDMA system, or a code in a CDMA system, for the duration of the communication with a mobile radio. Data to other users is not delivered over that dedicated channel, even if there are periods of silence in the communication when no data is being transmitted. Thus, although circuit-switched channels ensure minimal delay and a guaranteed bit rate, which is important for certain applications like voice communications, they are typically underutilized and also are usually limited in bandwidth. That limited bandwidth, while acceptable for certain applications like voice communications, is not well suited for many high speed data applications that require considerably more bandwidth.
Packet-based data communications are better suited for high speed data communications. Data packets are delivered individually using a “best effort,” packet-switched network like the Internet. Individual packet routing means that the bandwidth may be used efficiently and that higher bandwidth applications may be accommodated. While wireline data terminals, e.g., personal computers, are capable of utilizing higher, packet-switched network bandwidth, wireless data terminals are at a considerable disadvantage. The bandwidth of the radio interface separating the wireless data terminals from wireline, packet-switched networks like the Internet is limited.
Accordingly, considerable efforts are being made to increase the bandwidth for wireless data communication. That increased bandwidth is particularly important in the radio “down link” direction from the radio network to the mobile terminal. For example, a mobile terminal user might send in the radio “up link” direction, a low bandwidth request, e.g., a command, to download a web page from a site on the Internet. Downloading the web page and other information (especially graphics) from that web site requires considerably more bandwidth.
Thus, some current designs for cellular systems based on Code Division Multiple Access. (CDMA) are focusing different models for achieving high speed data rates on the radio downlink. It may be optimal to multiplex several low data rate channels (with transmissions made orthogonal in the code domain) and share the available base station transmitted power using some form of power control. But this approach is less optimal when a small number of high data rate users share that common bandwidth. Inefficiencies increase even further when the same bandwidth is shared by low rate voice and high rate data users. Accordingly, in some current CDMA designs, low rate data services such as voice are separated from high rate data services using adjacent, non-overlapping spectrum allocations. Using a dedicated portion of the spectrum, high data rate downlink packet transmissions, as shown in FIG. 1, are time-multiplexed and transmitted at full power with data rates and slot lengths varying according to user channel conditions. When the user transmission queues are empty, the only transmissions from the base station are those of short pilot bursts and periodic transmissions of control information. The pilot bursts allow the mobile terminals to estimate the current channel conditions over the downlink.
Another problem confronting data communications over the radio interface is the variable quality of the radio channel or link from base station to mobile terminal (downlink). The detected radio channel or link quality depends on a number of factors including the transmit power level, the distance between the mobile terminal and a transmitting base station in the radio network, interference from other transmitting base stations and mobile terminals, path loss, shadowing, short term multi-path fading, etc. If the channel quality is good, the base station may modify the signal transmission parameters to increase the data transmission rate from the base station to the mobile terminal. On the other hand, if the channel quality is bad, the signal transmission parameters may need to be adjusted to lower the data transmission rate to ensure that the signal is reliably received.
The process of modifying one or more signal transmission parameters to compensate for channel quality variations is sometimes referred to as “link adaptation,” where “link” refers to the radio link between a base station and a mobile terminal. Link adaptation may be accomplished by changing the transmit power of the base station, e.g., increasing the transmit power level for data transmitted to mobile terminals with a bad channel quality. Link adaptation may also be accomplished by changing the type of modulation and the amount of channel coding applied to the data to be transmitted by the base station. Moreover, link adaptation may be performed in the uplink by the mobile terminal.
Each base station may be divided into multiple sectors, where each sector serves a particular portion of the geographical area surrounding the base station. For example, each sector of a three sector base station serves approximately one third of the total geographical area surrounding that base station. Sometimes a base station or a base station sector is more generally referred to as an “access point” because it is a “point” where a mobile terminal may obtain access to the radio network.
The mobile terminal estimates the channel quality by measuring the signal quality of pilot signals or other broadcast signals transmitted by nearby “candidate” base station sectors, where some of the sectors may be associated with different base stations. Based on the estimated channel qualities, the mobile terminal determines a maximum data rate at which the mobile terminal can receive data for each base station sector and selects the sector with the highest data rate. The mobile terminal sends a rate/sector request message to one or more base stations in the radio network including information about a current estimate of a maximum supportable transmission rate as well as the currently requested sector to make the downlink transmission to the mobile terminal. That message also identifies a currently requested base station.
The performance of a link adaptation scheme depends on the accuracy of the signal quality measurement made by the mobile terminal. It is especially important that the signal quality measurements do not over-estimate the future signal quality. In the case of over-estimation, the link adaptation will select transmission parameters that are not sufficiently robust for the actual channel condition. Ideally, the goal of the mobile terminal is to accurately estimate a future radio channel condition at the time when the data packet transmission from the base station to the mobile terminal occurs. In other words, the mobile terminal should predict as accurately as possible the radio channel condition at some point in time in the future. A simple estimation technique is to measure the current signal quality at the mobile terminal of a signal received from the base station. Non-limiting examples of signal quality measurements include signal-to-noise ratio (Eb/No) and carrier-to-interference ratio (C/I). These current signal quality measurements are the estimates of the radio channel condition when the future packet transmission occurs. If the delay between the measurement and the actual packet transmission is sufficiently small, and if the actual signal quality measurements are accurate, this simple prediction technique is quite acceptable. For purposes of the following description, the measured signal quality is assumed to be carrier-to-interference ratio (C/I).
The carrier-to-interference ratio detected for a base station sector pilot signal received by a mobile terminal is affected by several factors which are generally divided into two groups: (1) the power of the signal whose quality is to be determined, and (2) the amount of noise and interference. If the interference from other transmitting base station sectors could be ignored, then the mobile terminal could simply decode each base station sector's pilot signal, and the accuracy of the signal quality measurement would be independent of the transmission of these other base station sectors. However, the transmissions from other base station sectors cannot be ignored. In fact, the interference detected by a mobile terminal may largely be attributable to other, non-selected base station sector transmissions.
Thus, in order to obtain an accurate signal quality measurement, it is desirable for all base station sectors to transmit at full power when the mobile terminal is measuring signal quality, regardless of the amount of data to transmit from each sector. Consider the signal quality estimation example shown in FIG. 2 for a synchronous, time division multiplex (TDM) radio communications system. In a synchronous TDM system, the time-multiplexed pilot symbols for each of the base station access points are transmitted at the same time as shown in FIG. 2. The mobile terminal can time its mi measurement to occur during a time period when all of the base station access points are transmitting their pilot signals.
If one or more base station access points does not transmit at full power when the mobile terminal measures the signal quality of the received pilot signal, the signal quality measurement may reflect a higher channel quality than what will actually exist when the data transmission occurs. Indeed, at the time the data transmission occurs, one or more inactive base station access points may have just started transmitting. One way of ensuring accurate signal quality estimates is to have all the base station access points transmit at full power during the pilot measurement time interval.
To ensure a “worst-case” signal quality estimate (i.e., correct or underestimated signal quality), all of the base station access points must be synchronized so that all mobile terminals know that all base station access points are active during the measurement time period. Unfortunately, in asynchronous radio communications, it is unlikely that all base station access points, e.g., sectors, are actively transmitting during the same measurement time interval. A simplified, asynchronous TDM example for two base station access points is shown in FIG. 3, where the time-multiplexed pilot and data time slots for each access point are offset from each other. If the signal quality measurement is made by the mobile terminal during a first time period M1, both base station access points are transmitting at full power for both pilot and data. Therefore, an accurate estimate of the future signal quality, (e.g., C/I), when the information will be transmitted is obtained. On the other hand, during measurement time interval M2, the access point 1 is not transmitting at full power for the full measurement time interval M2. Therefore, a more favorable, and possibly incorrect, signal quality estimate is detected by the mobile terminal.
FIG. 4 shows a simplified example where the pilot and user data in each time slot are code multiplexed. The pilot is a fraction, e.g., 10%, of the total base station power and the user data is transmitted with the remaining power. In this case, when only the pilot is transmitted, as shown in measure time interval M2, only a small percentage, e.g., 10%, of the base station transmit power is detected as interference by a monitoring mobile terminal.
One way of addressing this problem of inaccurate signal quality measurements in both synchronous and asynchronous systems is for all base station access points to transmit at full power continuously. This approach is illustrated for a TDM-type system in FIG. 5. If there are no data symbols to be sent, “dummy” data symbols are transmitted in the data field. This approach works as well in a CDM-type system shown in FIG. 6. The dummy data bits transmitted in the second time slot for base station access point 2 ensure that full transmit power is detected during measurement time intervals M1 and M2. In this way, the signal quality measurement by a mobile terminal is assured to be a worst case estimate over each time slot.
A downside with all access points continuously transmitting at full power is the waste of resources. Although fewer resources are wasted when the system is heavily-loaded and dummy symbols are rarely transmitted, this is not the case during low traffic periods. In addition to increasing power consumption, such full power transmission unnecessarily increases the general interference level (which reduces overall system capacity) as well as cooling needs at the base station.
These problems are avoided by the present invention. The transmission load between base station and mobile terminal is taken into account in controlling transmit power levels. This transmission load power control is implemented so that the accuracy of channel quality estimates is not significantly affected. While in a preferred example embodiment, the transmission load is in the downlink direction and the base station transmit power level is controlled, the invention may be employed in the opposite, uplink direction to implement mobile terminal power control.
The present invention employs a power control methodology that adapts to the transmission load associated with communications between a base station and a mobile terminal. The base station gradually adjusts the power at which data is transmitted to the mobile terminal based on that associated transmission load. As a result, radio channel quality measurements influenced by that base station data transmission to the mobile terminal are not significantly affected by the transmit power adjustment. In one example implementation, the rate at which the transmit power is changed is slower than the rate at which mobile stations measure channel quality. For example, mobile terminals may detect a signal-to-interference ratio every time slot and use that quality measurement for purposes of selecting a maximum transmission rate for the next time slot. The transmit power might be changed by an incremental amount once every ten time slots. In another example implementation, an even smaller incremental change is made every time slot. In both implementations, gradual changes to the base station transmit power do not significantly affect the accuracy of the mobile terminal channel quality estimates. Moreover, if there is a relatively low transmission load, the base station does not waste resources or generate unnecessary interference by transmitting at maximum power.
In a preferred, non-limiting, example implementation, a base station includes plural sectors, each sector includes signal processing and radio transceiving circuitry, a transmit buffer for storing packets to be transmitted to a mobile terminal, and data processing circuitry that adjusts the power at which the data packets are transmitted to the mobile terminal. That adjustment of transmit power is based upon the transmission load, e.g., number of packets to transmit, over a predetermined time period associated with the communication between the base station and mobile terminal. In one example implementation, the predetermined time period is preferably greater than one time slot during which the packet information is sent. In another example implementation, the predetermined time period may correspond to one time slot, but the amount of adjustment to the transmit power is relatively small.
In one non-limiting example, an average transmission load is determined based on a current amount of packet data stored in the transmit buffer corresponding to the mobile terminal, and an earlier amount of packet data previously detected and transmitted from the transmission buffer. If the current amount of data exceeds the previously transmitted amount of data, the transmit power is increased by an incremental amount (e.g., up to a maximum value). Conversely, if the current amount of data does not exceed the amount of data previously sent, the transmit power is decreased by an incremental amount (e.g., down to a minimum value). The adjustment of the base station sector transmit power may occur less frequently than the frequency at which mobile terminals determine the signal quality of base station transmissions. Alternatively, the incremental amount of change is made even smaller for more frequent power changes.